In less than a decade, the deadly bat disease called white-nose syndrome has taken hold across the eastern half of the United States and up into Canada. With the disease confirmed in Michigan and Wisconsin in April, WNS has now been documented in 25 U.S. states and five Canadian provinces. Millions of hibernating bats from six species have been infected with the fungus Pseudogymnoascus destructans, or Pd.

The disease attacks a bat’s skin, and an infected bat often displays abnormal behavior, such as heading for the mouth of a cave in winter when it should be tucked away, safe and warm, far from the entrance. Such behavior is thought to be one reason behind infected bats’ loss of fat reserves, emaciation and death.

Since scientists discovered the epidemic, they’ve been struggling to figure out how to prevent its spread. One problem is diagnosis. The “gold standard” for diagnosing white-nose syndrome has been to euthanize a bat, then take samples of its skin and stick them under a microscope. But this is time-consuming, and it requires special facilities — and the death of the bat.

However, researchers have just found a new tool for diagnosing WNS — ultraviolet light. Shining UV light with a wavelength between 368 and 385 nanometers causes the Pd fungus to fluoresce orange-yellow, Gregory G. Turner of the Pennsylvania Game Commission and colleagues report May 22 in the Journal of Wildlife Diseases.

Finding fungal infections with UV light is actually a tried and true technique, used to diagnose tinea capitis (also known as ringworm) in people, for example. Scientists aren’t sure why Pd lights up under UV light, but they think that once Pd filaments, called hyphae, penetrate the bat’s skin, those hyphae secrete substances that eat away at the living tissue and also happen to fluoresce under UV light.

In the new study, the researchers first tested their idea on the wings of 168 bats that had been submitted to the U.S. Geological Survey National Wildlife Health Center from March 2009 to April 2012. Of those bats, 80 fluoresced under UV light, and 79 of 80 were confirmed to have WNS. All 88 bats that did not glow yellow-orange tested negative for the fungus.

The method got a test out in the field in 2012 in the Czech Republic, where WNS has been found but without the devastating mortality that has accompanied its spread in North America. Twenty-one of 22 bats that fluoresced tested positive for Pd in the lab; all 40 bats that didn’t have any orange-yellow spots under the UV light tested negative.

The UV light method won’t replace other tests for WNS syndrome, the researchers admit. It probably won’t detect a bat with a mild case of the disease, and an inexperienced observer (or one without great vision) may miss infected bats. But, they write, “the ability to perform targeted and nonlethal sampling of bats for WNS offers a needed tool to facilitate enhanced surveillance and research for the disease.” And that can only be a positive thing in the efforts to stem the spread of the deadly WNS.